In a base station apparatus in a mobile communications system using, e.g., a W-CDMA (Wideband Code Division Multiple Access) scheme as a mobile communications scheme, signals need to be greatly amplified by an amplifier to be delivered as wireless signals to a distant place where a mobile station apparatus is installed. However, since the amplifier is an analogue device, input and output characteristics thereof behave nonlinearly. Specifically, beyond an amplification limit referred to as “saturation point,” an output power remains constant even if an electric power inputted to the amplifier is increased. Further, this nonlinear output characteristic gives rise to a nonlinear distortion. In a signal to be sent, signal components outside of a desired signal frequency range are suppressed to a low level before the amplification. However, after passing through the amplifier, the signal to be sent is nonlinearly distorted, so that some signal components therein are leaked out of the desired signal frequency range into adjacent channels. Since the output power of the signal to be sent is high in, for example, a base station apparatus as described above, the leakage power into the adjacent channels is regulated strictly. Therefore, it is an important technical problem how to reduce the adjacent power leakage power.
As a technique for compensating the distortion caused by the nonlinear characteristics of the amplifier and enlarging an operation range of the amplifier, a predistortion method for compensating the distortion is widely used. FIG. 2 illustrates a block diagram depicting a schematic configuration of a conventional amplifier using the predistortion method for compensating the distortion. A power detection unit 201 detects an electric power or an amplitude of an input signal SIN or a value of a logarithm thereof. In the following, “input level L” refers to an instantaneous value (in a baseband range) or an envelope-(after the signal is modulated by a carrier) of the detected power or amplitude. A distortion compensation table 202 stores compensation values M corresponding to the input level L at corresponding addresses, and outputs a compensation value M corresponding to a specific input level to a predistortion unit 203. The predistortion unit 203 performs a predistortion on the input signal SIN by using the compensation value M, and outputs a predistorted signal to an amplifying unit 204. Then, the amplifying unit 204 amplifies the predistorted signal, and outputs the amplified signal as an output signal SOUT. Herein, if the predistortion performed on the input signal SIN by using the compensation value M has a characteristic opposite to that of the nonlinear distortion in the amplifying unit 204, the output signal SOUT becomes equivalent to a signal obtained by amplifying the input signal SIN by a linear circuit, so that the nonlinear distortion in the amplifying unit is compensated.
Further, if a temperature or a source voltage changes, the input and output characteristics of the amplifying unit 204 change, and thus the nonlinear characteristics thereof also change. A control unit 205 monitors the output signal, and updates the compensation values if the characteristics of the amplifying unit change, thereby updating the distortion compensation table.
FIG. 2 depicts schematic configuration of a conventional amplifier using the predistortion method for compensating the distortion. In case where the base station apparatus employs an orthogonal modulation such as 16 QAM (Quadrature amplitude Modulation), the input signal SIN includes an I (In-phase) signal and a Q (Quadrature) signal in the baseband, the distortion compensation table 202 and the control unit 205 are configured as digital circuits, and the amplifying unit 204 is operated in a wireless frequency range. Further, the I and Q signals in the baseband are converted into wireless signals by a D/A converter, an orthogonal modulator, and a frequency converter and the like that are not shown in FIG. 2, and then inputted into the amplifying unit 204. The predistortion unit 203 includes a processing circuit for processing the I and Q signals in the baseband or compensation circuit for compensating an amplitude and a phase of an analog signal after the orthogonal modulation. Further, if necessary, a frequency converter, an orthogonal demodulator, a D/A converter and the like may be installed between an output terminal of the amplifying unit 204 and the control unit 205.
As prior arts, References 1 to 3 disclose the predistortion method for compensating distortions described above. Among these, Reference 1 discloses a “distortion compensation circuit and method,” proposing a technique for updating the distortion compensation table quickly and easily when adaptive information such as temperature, source voltage and frequency is changed. Reference 2 discloses a “transmitter” for performing the distortion compensation more precisely by using a DC offset generated in a D/A converter and an orthogonal modulator for converting the baseband I and Q signals into wireless signals. Further, Reference 3 discloses a “radio equipment and distortion compensation method” for performing the distortion compensation more precisely by using a DC offset generated in an orthogonal modulator.
Reference 1: Japanese Patent Laid-Open Application No. 2002-26998
Reference 2: Japanese Patent Laid-Open Application No. 2001-237723
Reference 3: Japanese Patent Laid-Open Application No. H10-65570
Since the characteristics of the amplifying unit change as the temperature thereabout, the source voltage and the like change, the distortion compensation table has to be updated in response to the change in the environmental conditions. As an updating method for updating the distortion compensation table, there is widely used a perturbation method, in which signal components outside of the desired signal frequency range in the output signal of the amplifying unit are monitored, and if any of them exceeds a preset level, the distortion compensation values are minutely changed in a manner to reduce the signal components outside of the desired signal frequency range. However, if the distortion compensation table is updated by using this method, an average power of the amplifying unit output is sometimes changed by the predistortion reflecting the updated results, thus causing an undesirable effect to a transmission apparatus.